Borehole seismic surveys are conducted for the purpose of mapping subsurface geologic structures. In a typical borehole seismic survey, a source is placed in a borehole at a selected depth and excited in order to produce acoustic waves in the adjacent geological formations. Sensors are usually placed at the earth's surface or at selected depths in the same or another borehole in order to detect the acoustic signals after they have propagated through and been reflected from geological formations.
During the seismic surveys, acoustic waves generated by the source also propagate vertically in the receiver borehole in the form of tube waves. These tube waves are generated when incident energy encounters the top and bottom of the borehole and/or any impedance contrasts along the length of the borehole. The tube waves interfere with the acoustic signals in the borehole and surrounding formation. These tube waves are also detected by the sensors. A problem in borehole seismic surveys is distinguishing the desired acoustic waves detected by the sensors in the borehole from the undesired tube waves.
Some efforts have been made to solve the problem of tube waves interfering with the desired acoustic waves. One conventional approach to solving this problem has been to use inflatable bladders in the source borehole. These bladders are placed below and possibly above the source as disclosed in a Chelminski patent, U.S. Pat. No. 4,858,718. Bolt Technology Corporation implements the method described in the Chelminski patent. The bladder is made of rubber or a polymeric expandable material and uses a compressed gas such as air or nitrogen to fill the bladder downhole. This device must use a pressure regulator valve and relief valve to control the pressure inside the bladder. The bladder maintains near equilibrium with the borehole fluid pressure. Also, the device must have either a compressed gas canister or a surface compressor with an umbilical cable down to the bladder to inflate the bladder. Bolt's European Patent Application No. 88103279.1 states that its suppressor has an axial length of at least "about 5 times its external diameter" of the device which would make its length less than 5 feet.
Gildas U.S. Pat. No. 4,817,755 discloses a device designed to couple source energy to the borehole wall in downhole shooting. The device uses metal canisters above and below an impulsive source. These canisters are filled with a plastic foam material to help attenuate the vertical component of the seismic energy. The canisters must be remotely controlled from the surface. Also, these canisters spool out primacord that is used as a source.
Both of the aforementioned approaches use mechanical devices to attempt to minimize tube wave energy. To avoid the complexity and unreliability of having mechanically or pneumatically activated devices downhole, a means of tube wave suppression is needed in either or both the source and receiver holes that will be cost effective, reliable and easy to implement.